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Actually, you can't.
Check the Hive Recovery Guide.
The code can be audited and the webpage can be run locally

Okay, thank you, I will check that later.
If the webpage can be run locally without internet connection that is at least something, I guess...

Do you mean it is not possible to create something that only uses keychain for changing the recovery account?

The webpage can run locally, but not without an internet connexion.

Do you mean it is not possible to create something that only uses keychain for changing the recovery account?

Yes, your private owner key is mandatory to issue the change_recovery_account operation to the blockchain and keychain does not store the owner key.